Patrick Wall

PAIN

The Science of Suffering

Patrick Wall is a neuroscientist who is best known for the very
influential gate theory of pain which he described in collaboration with
Ronald Melzack. He was a co-inventor of transcutaneous nerve
stimulation, which has proved a valuable method of pain relief for many
patients. He is one of the foremost authorities on pain, and has helped
to bring about a radical restructuring of our understanding of its
neurophysiological basis. In this book he presents his latest thoughts
on the subject for a general audience without specialist medical or
scientific knowledge.

By way of introduction he adduces some puzzling facts about pain. There
is no simple correlation between the severity of an injury and the
degree of pain experienced by the patient; indeed, some severe injuries
produce no pain at all at the time. The complexity of the phenomenon is
illustrated by a number of case histories involving humans and also
animals. The implications of these apparent paradoxes form the basis of
the discussion in the rest of the book.

Pain, Wall insists, is not a simple matter of transmitting impulses
along nervous pathways to some putative pain centre in the brain. There
is no such centre. Modern techniques of imaging have shown that many
brain areas are involved, including the frontal lobes, motor and sensory
areas, anterior cingulate cortex, hypothalamus, midbrain, and
cerebellum. Thus pain perception is a response of the whole brain, not
of any one area.

The involvement of motor areas is particularly remarkable.
Neuroscientists who adhere to classic views of pain think that this can
be explained by saying that sensation occurs first, and is then followed
by motor planning and the beginnings of movement. Wall, however,
believes that this sequential idea may be a fundamental error. He
suggests that there is no separation between the sensory brain and the
motor-planning brain, and he goes on to describe a new hypothesis about
pain based on these recent findings.

On Wall's hypothesis, attention is critical in pain perception. This
explains the paradox that the degree of injury is unrelated to the
severity of pain; everything depends on where we focus our attention. It
also explains how hypnosis can prevent pain, because in the hypnotic
state the attention is directed elsewhere. It is now possible to
visualize the process of directing attention by imaging the brain. When
pain is perceived, the anterior cingulate cortex becomes activated
first, which is exactly what would be expected if attention is the key.

The classic theory says that the brain analyses the sensory input to
determine what has happened and presents the pain as a pure sensation,
after which what one might call an executive decision is taken regarding
what to do about it. Wall's theory is that the entire brain, and indeed
the whole body, are involved from the outset. There is no such thing as
a pure pain; pain is always part of a motor response, or at least a
planned motor response (changes occur in the motor system of the brain
even when no overt muscular contractions ensue). This is a radical but
illuminating idea, with far-reaching implications for those of us who
are involved in trying to relieve pain in patients. It also has
implications for philosophical problems such as the nature of
consciousness itself.

On the practical level, Wall makes well-justified criticisms of the
pseudo-diagnoses that patients suffering from pain are often offered.
The cause of back pain is unknown in 85 per cent of cases, yet there is
no lack of therapists who will give confident explanations ('slipped
disc', or 'arthritis') to every patient they see. (To be fair to the
therapists, patients often demand explanations of this kind and may feel
insecure if the therapist professes ignorance.) 'Trapped nerves' is
another pseudo-diagnosis; it is intuitively appealing, largely because
it is conceptually simple, but although freeing nerves surgically
sometimes gives relief, there is little evidence for nerve damage in
such cases. In the case of Morton's metatarsalgia, a painful disorder of
the foot which is supposed to be due to trapping of nerves between the
ends of the bones in the forefoot, the surgeon removes the "trapped
nerve" and this may help, yet when the tissue that is removed is
examined it is rare to find any nerve material at all.

So why does treatment sometimes work in such cases? Perhaps the benefit
comes simply from disturbing the tissues; this may be true for many
kinds of surgical intervention for pain, including disc surgery and the
operations used to treat trigeminal neuralgia. Wall is surely right to
say that just because removing some tissue relieves pain, it does not
follow that the tissue in question was causing the pain originally.

Pain is a fascinating subject theoretically and also of the utmost
importance to us all, since we all experience it at some time and some
of us suffer it constantly and severely. This book presents a lot of
facts and interpretations which will be new to many people, health
professionals and patients alike, and it is therefore to be welcomed.
Unfortunately, however, Wall has not been well served by his editors.
Minor and not so minor typos are plentiful. Some are obvious, such as
'complimentary' for 'complementary', but others are more subtle. It took
me a couple of minutes to work out that the 'body hole' between each
pair of vertebrae, containing the dorsal root ganglion, was really the 'bony
hole'; would a reader without anatomical knowledge ever understand what
was meant? And the date of the arrival of acupuncture in England is
given as 1685 which is said to be in the Elizabethan age.

But it isn't only typos and historical errors that are the problem.
There are a number of places where the sense is hard to follow, not
because it it is over-technical, but because the editing has been
careless. Reading the chapter on the philosophy of pain I gained the
impression at first that Wall was defending dualism. This was obviously
out of keeping with the rest of his approach and I did in the end manage
to disentangle his meaning, but I was left with the feeling that the
issue hadn't been given enough space to allow it to be properly aired. I
had the same feeling about the discussion of the placebo phenomenon and
how it relates to Wall's pain theory, which is a pity, because it is
important and deserved a fuller treatment than that allowed to it here.

Much of the difficulty I found with this book stems, I think, from the
fact that it is pitched at the wrong level. It touches on numerous
fascinating ideas but they needed more elaboration than they get here.
Probably the editorial policy was to avoid putting off readers with no
knowledge of science or medicine, but I think it would have been better
to have at least included some appendixes in which more detailed
information was given.

As it is, no references are provided for the studies mentioned in the
text, so that even if one wishes to look up the sources it is difficult
to do so. Indeed, in the case of one really remarkable piece of research
it's impossible. In a recent study it was found that stimulation of the
vestibular system of the inner ear could temporarily restore normal body
perception to patients who were neglecting one side of their bodies
after a stroke. This is indeed astounding, as Wall says, and I'm sure
that many readers would wish to read the original paper; but all we are
told about the authors is that they were Italian doctors in Milan.

In his preface, Wall says that this is not supposed to be a textbook,
and he refers readers seeking for more detail to his 1996 book 'The
Challenge of Pain', written in conjunction with Melzack. However, the
present book touches tantalizingly on a lot of new material, which
deserves a fuller discussion than it gets here.